1. Field of the Invention
The present invention relates to a zoom lens system. More particularly, the present invention relates to a zoom lens system suitable for use in a lens-shutter single-lens-reflex camera.
2. Description of the Prior Art
In conventional lens-shutter cameras, the taking optical system and the viewfinder optical system are typically provided as separate optical systems. For this reason, such lens-shutter cameras suffer from parallax occurring between the taking optical system and the viewfinder optical system. In recent years, as lens-shutter cameras have come to be equipped with zoom lens systems having higher and higher magnifications, increased parallax and increased size have become serious problems with lens-shutter cameras.
On the other hand, in single-lens-reflex cameras, the taking optical system is used also as an objective lens of the viewfinder optical system. Accordingly, no parallax occurs between the taking optical system and the viewfinder optical system. However, in typical single-lens-reflex cameras, a swing mirror is provided behind the taking optical system, and the use of the swing mirror inevitably makes such cameras larger. The reasons why the swing mirror makes cameras larger are as follows. First, since the swing mirror is placed behind the taking optical system, it sets a limit on the back focus. Second, since the swing mirror is placed far away from the aperture diaphragm that is placed in the middle of the taking optical system, it needs to be large enough to reflect a light beam having a beam diameter corresponding to the position where the swing mirror is placed.
To prevent parallax and reduce the size of cameras, U.S. Pat. No. 4,730,199 proposes a zoom lens system consisting of a positive, a negative, a positive, and a negative lens unit. Specifically, in this zoom lens system, an incoming light beam is split into two light beams, one for photographing and the other for the viewfinder, by a beam splitter provided between the third and fourth lens units. This construction not only eliminates parallax, but also allows cameras to be made smaller, since it does not require a swing mirror.
Moreover, to prevent parallax and reduce the size of cameras, U.S. Pat. No. 5,272,565 proposes another zoom lens system. Specifically, in this zoom lens system, an incoming light beam is split into two light beams, one for photographing and the other for the viewfinder, by a reflecting means provided within one of the zoom lens units. This construction not only eliminates parallax, but also allows cameras to be made smaller, since it does not require a swing mirror.
Moreover, to prevent parallax and reduce the size of cameras, Japanese Published Patent No. 6-44098 proposes still another zoom lens system. Specifically, in this zoom lens system, an incoming light beam is split into two light beams, one for photographing and the other for the viewfinder, by a beam splitting member provided between the zoom lens units. This construction not only eliminates parallax, but also allows cameras to be made smaller, since it does not require a swing mirror.
However, in the zoom lens system proposed in U.S. Pat. No. 4,730,199, the paths of off-axial light rays between the third and fourth lens units are extremely far from those of axial light rays, and accordingly it is necessary to use a large beam splitter in order to cover all light beams. As a result, even if the viewfinder optical system is made smaller with the view to making a camera smaller, the large beam splitter spoils the compactness of the camera. Therefore, with this zoom lens system, it is difficult to realize a compact camera.
Moreover, in the zoom lens system proposed in U.S. Pat. No. 4,730,199, the zoom lens unit disposed just in front of the beam splitter has a positive power, and accordingly off-axial light rays incident on the beam splitter form large angles with respect to axial light rays. This not only requires a larger beam splitter, but also requires that the optical systems disposed behind the beam splitter (i.e. the negative fourth lens unit and the viewfinder optical system) have accordingly larger diameters, since off-axial light rays diverge steeply after being split by the beam splitter. As a result, the increased size of the beam splitter and the optical systems disposed behind it spoils the compactness of the zoom lens system and the viewfinder optical system. Therefore, with this zoom lens system, it is difficult to realize a compact camera.
In zoom lens systems, zooming is preferably performed by moving zoom lens units individually, because this makes it easy to correct aberrations properly. However, in a zoom lens system in which a beam splitter is provided between the zoom lens units, such as the one proposed in U.S. Pat. No. 4,730,199, extraction of a light beam for the viewfinder requires a complicated construction of the lens barrel precisely because the zoom lens units move individually. Moreover, incorporation of part of the viewfinder optical system into the lens barrel of the zoom lens system requires an even more complicated construction of the lens barrel, and thus requires that the lens barrel have a larger diameter.
In the zoom lens system proposed in U.S. Pat. No. 5,272,565, a reflecting means is provided within one of the zoom lens units. Such arrangement of a reflecting means within one of the zoom lens unit requires that one zoom lens unit be divided into two zoom blocks, one in front of the reflecting means and the other behind it, within the lens barrel. This not only complicates the construction of the lens barrel, but also makes it difficult to maintain the optical performance of that zoom lens unit in the process of assembly.
Moreover, in the zoom lens system proposed in U.S. Pat. No. 5,272,565, focusing is performed by moving part of the lens elements constituting the zoom lens unit that includes the reflecting means, or by moving the first lens unit. The former focusing method requires that part of the lens elements constituting the zoom lens unit that includes the reflecting means be moved for focusing. This further complicates the construction of the lens barrel, and thus makes it more difficult to maintain the optical performance of that zoom lens unit in the process of assembly. The latter focusing method, as compared with other focusing methods, requires a longer distance for the movement of the focusing lens unit, and thus requires a larger space for focusing. This is disadvantageous to the construction of the lens barrel.
In a zoom lens system in which a beam splitting member is provided between the zoom lens units, such as the one proposed in Japanese Published Patent No. 6-44098, the zoom lens units disposed in front of the beam splitting member and those disposed behind it tend to act independently as two separate groups. This means that aberrations need to be corrected properly within each of these groups. However, in the zoom lens system proposed in Japanese Published Patent No. 6-44098, since only one zoom lens unit is provided behind the beam splitting member, it is not possible to correct aberrations satisfactorily.
The zoom lens system proposed in Japanese Published Patent No. 6-44098 has only one zoom lens unit behind the beam splitting member. In a zoom lens system like this, in which only one zoom lens unit is provided behind the beam splitting member, the zoom lens units disposed in front of the beam splitting member need to correct most of the aberrations occurring in the zoom lens system. Since a viewfinder optical system generally needs to comply with more limitations than a taking optical system, it is difficult, in a construction in which the zoom lens units disposed in front of the beam splitting member need to correct most of the aberrations, to make those lens units suitable for a viewfinder. This, therefore, sets more limitations on the arrangement of the viewfinder optical system.